A type in which a polyimide film called a cover ray film is punched by means of a die produced fitting to a pattern and then adhered with an adhesive and a type in which an overcoat agent of a UV ray-curing type or a heat-curing type provided with a flexibility or a thermosetting type is coated by a screen printing method have so far been available as a surface protecting film for flexible wiring circuits, and particularly the latter is useful in terms of workability.
Resin compositions comprising mainly epoxy resins, acrylic resins and combined resins thereof are known as the overcoat agents of the above curing type. They comprise resins modified by introducing particularly a butadiene skeleton, a siloxane skeleton, a polycarbonate skeleton, a long-chain aliphatic skeleton and the like as a principal component in many cases. This has enhanced a flexibility and inhibited warping caused by curing shrinkage and heat shrinkage from being brought about while suppressing as much as possible a reduction in a heat resistance, a chemical resistance and an electric insulating property which are essentially imparted to a surface protective film.
In recent years, however, a reduction in a weight and a size of electronic equipments promotes as well a reduction in a thickness of flexible substrates, and influences of a flexibility and curing shrinkage of an overcoated resin composition has come to be exerted more notably. Accordingly, an overcoat agent of a curing type can not satisfy required performances in terms of a flexibility and warpage caused by curing shrinkage. Consequently, various researches are made in order to solve the above problems in the existing situation.
For example, patent document 1 discloses a polyamideimide resin prepared by reacting trimellitic anhydride with both end-diisocyanate polyurethane obtained by reacting a diisocyanate compound with polycarbonatediol compound comprising diol having 6 or less carbon atoms as a raw material. However, a cured matter thereof has the defect that it is inferior in a flame retardancy and is not satisfactory in terms of long term reliability in electric characteristics.
Further, such thermosetting compositions are required to satisfy various physical properties according to uses. In particular, when they are used for electronic parts, a flame retardancy is required as an important physical property in addition to a heat resistance, an insulating property, a flexibility and the like, and if the flame retardancy is low, the uses thereof result in being limited.
However, it has not been easy to endow conventional thermosetting compositions with a sufficiently satisfactory high flame retardancy. Methods using halide base flame retardants such as brominated epoxy resins and flame retardants obtained by combining the above flame retardants with flame retardant auxiliaries such as antimony trioxide have so far been available as a method for providing a flame retardancy (patent document 2, patent document 3 and the like). However, the above flame retardants are inferior in reliability under high temperature environment in a certain case. Further, brominated epoxy resins have involved the problem that blending an amount thereof in which a satisfactory flame retardant effect is obtained damages a flexibility of the resin compositions. In recent years, originating with a dioxin problem, the move of restriction is involved as well in halogenated resins including decabromoether. Non-halogen and non-antimony are requested as well to resin molding materials used for electronic parts. An improvement in a flame retardant effect by blending flame retardants of a conventional type has been limited from the above point of view.
Further, resin compositions using phosphoric esters as a flame retardant are proposed as well (patent document 4, patent document 5, patent document 6 and the like). However, the compositions merely blended with the phosphoric esters has a weak flame retardant effect and can not sufficiently satisfy the standard of a flame retardancy in a UL standard. Accordingly, thermosetting compositions which can achieve non-halogen and non-antimony and which are provided with a higher flame retardancy are desired to be developed.
A phosphorus-containing compound and a resin composition containing the above compound are described in patent document 7, and a phosphorus-containing urethane (meth)acrylate compound having a phosphorus atom in a skeleton and a resin composition containing the above compound are described in patent document 8. It is described as well that the respective compounds exhibit a flame retardant effect. Among them, a cured matter formed from the resin composition described in the patent document 8 has a satisfactory flame retardancy, but it is required to be further improved in an insulation resistant characteristic.
Dialkylphosphinic acid salts are described as flame retardants in patent document 9, patent document 10 and patent document 11, but examples in which resists have successfully been turned into flame retardation are not described therein.                Patent document 1: JP A-2004-137370        Patent document 2: JP A-1997-325490        Patent document 3: JP A-1999-242331        Patent document 4: JP A-1997-235449        Patent document 5: JP A-1998-306201        Patent document 6: JP A-1999-271967        Patent document 7: JP A-2005-290134        Patent document 8: JP A-2003-212954        Patent document 9: JP A-2005-325358        Patent document 10: JP A-2002-284963        Patent document 11: JP A-2006-28249        